1. Field of the Invention
The subject invention generally pertains to the control of air conditioners and heat pumps that have a direct-expansion evaporator (DX evaporator), and the invention more specifically pertains to maintaining the refrigerant leaving the evaporator at a desired minimal level of superheat.
2. Description of Related Art
Many refrigerant systems (chillers) have a DX evaporator in which a refrigerant absorbs heat while expanding from a liquid to a gaseous state directly inside the evaporator. The absorbed heat can cool air supplied to a comfort zone or cool an intermediate fluid such as chilled water. If the chiller functions as a heat pump, heat absorbed by the evaporator can be released to the comfort zone by way of a condenser.
The heat transfer coefficient across the tube walls of a DX evaporator is generally greatest when the refrigerant inside the tubes is saturated, partially liquid, rather than superheated to a gas. Liquid refrigerant, unfortunately, can damage a compressor, which draws the refrigerant from the evaporator. So ideally, the refrigerant enters the DX evaporator as a liquid and is not completely vaporized until just prior to leaving for the inlet of the compressor.
To this end, expansion valves, which controllably feed refrigerant from the condenser into the evaporator, are controlled so as to achieve a desired minimal amount of superheat within the evaporator. Examples of superheat-related controllers are disclosed in U.S. Pat. Nos. 4,505,125; 4,523,435; 4,527,399; 5,067,556; 5,187,944; 5,987,907 and 6,032,473. There is a common problem, however, facing perhaps all superheat-related controllers.
During steady state operation near a desired minimal superheat condition, the expansion valve controller preferably has a relatively low gain or response, as a slight adjustment to the opening or closing of the expansion valve can have a dramatic effect on the degree of superheat. The chiller, however, may not always be operating at this optimum steady state condition. Although a slight movement of the expansion valve can produce an appropriate change in superheat when operating just above the desired saturation point, that same amount of movement in opening may be insufficient when operating at greater levels of superheat. Thus, an expansion valve “tuned” for optimum response when operating at slightly above saturation may be too sluggish under conditions of greater superheat or no superheat (in saturation).
One conceivable solution may be to attempt identifying the nonlinear relationship between the amount of superheat and the opening of the expansion valve and adjust the response of the valve accordingly. The nonlinear relationship, however, is not necessarily a static relationship, particularly in cases where the chiller has varying load capability. Many systems vary the load by selectively unloading a compressor, selectively operating multiple compressors, selectively energizing multiple evaporator fans, varying the speed of an evaporator fan, etc. A controller could monitor such load-varying events and try to adjust the expansion valve's response accordingly, but such an approach becomes a daunting challenge, as the effect that each of these events has on the superheat needs to be accurately quantified, not only for when the events occur alone but also when they occur in various combinations with each other.
Consequently, a need exists for a better method of controlling the operation of an expansion valve to maintain a desired minimal level of superheat over widely varying load conditions.